1. Field of the Invention
The present invention relates generally to techniques for permitting conversations between multiple parties at different locations, and more particularly to techniques for reducing echo effects while maintaining a more natural flow of conversation.
2. Description of Related Art
When electronic systems are utilized to communicate sounds between two or more locations, A, B, C, etc, undesirable echoes are generated when sounds received at location A, from locations B or C or others, are permitted to excite the microphone at location A and contribute to the audio signals sent out from location A. Methods used to reduce this echo effect include half duplex operation and acoustic echo cancellation.
In an audio communications system, which permits receiving and sending at the same time, any sound coupled from an output sound transducer, e.g., a loud speaker to an input sound transducer, e.g., a microphone, causes an echo-like disturbance at the far end unless the echo-like disturbance is electronically suppressed. One method used for echo suppression either reduced the strength of the signal being sent to the loud speaker or the signal being sent to the communications medium in response to a determination of the presence of locally generated sound. As described more completely below, this type of echo suppression typically resulted in loss of information.
Since strong attenuation was usually required to suppress this echo generation, the strong attenuation effectively resulted in one directional communications at any given time, thus resulting in half duplex operation. Inevitably, the users cut off the signal from each other with annoying frequency.
When the received sound is active and the locally generated sound is active, the condition is called “double talk”. In half duplex operation during double talk, one or the other signal direction was cut off even though that direction was active.
A common example of this occurs with office speaker phones. These annoyances are greatly exaggerated when the communications medium introduces significant or variable delays as is common using the Internet and as is unavoidable in extraterrestrial travel.
Thus, in half duplex operation, the signal flowing in one direction is temporarily shut off when the strength of the signal flowing in the opposite direction exceeds some threshold value. This has the disadvantage that potentially important portions of the signals are lost.
To avoid the annoying characteristics of half duplex operation some implementations resorted to very complicated modeling of the acoustic coupling between the loud speaker and the microphone. Using this model an estimate was formed for the signal components due to acoustic coupling and that estimate was subtracted from the microphone signal before the signal was sent out. Such techniques routinely resorted to multiple heuristic and non-linear processes to mask the remaining echoes and other distortions resulting from imprecise model parameters and imperfect transducers.
In the operation of an acoustic echo canceller, a model of the coupling between the loud speaker and the microphone was trained adaptively during operation to subtract an amount of the signal level of the loud speaker from the microphone signal in an attempt to remove or reduce the perceived echo. Many training schemes and nonlinear methods were combined in acoustic echo cancellation to accommodate changes in the environment and imperfect estimates in the mathematical model. Some training schemes utilized strong bursts of noise to facilitate the rapid convergence of the mathematical model. Acoustic echo cancellation has the disadvantages of high cost and poor performance at unpredictable times.